Metal-Oxide-Semiconductor (MOS) devices are basic building elements in integrated circuits. Recent development of the MOS devices includes forming replacement gates, which include high-k gate dielectrics and metal gate electrodes over the high-k gate dielectrics. The formation of a replacement gate typically involves depositing a high-k gate dielectric layer and metal layers over the high-k gate dielectric layer, and then performing Chemical Mechanical Polish (CMP) to remove excess portions of the high-k gate dielectric layer and the metal layers. The remaining portions of the metal layers form the metal gates.
In conventional formation methods of the MOS devices, the threshold voltages of the MOS devices may be changed by performing a thermal anneal process when conducting ammonia to treat the high-k dielectric layers. Although the threshold voltage can be changed, it was impossible to adjust the threshold voltages to intended values, and further adjustment had to be achieved by adopting different work-function metals and adjusting the thickness of the work-function metals.